1. Field of the Invention
The present invention relates to a Doherty amplifier which combines outputs of a carrier amplifier and a peak amplifier that are different in operation class from each other, to deliver the combined output.
2. Description of the Related Art
Power amplifiers utilized in radio communications systems are required to provide linearity and high efficiency.
Also, in recent communications systems which employ multi-value digital modulation, often handle signals having a signal amplitude whose average value is different from a maximum value of the same. For amplifying such a signal, in a conventional amplifier, an operating point of the amplifier must be set such that the signal can be amplified without distortions up to a maximum amplitude. For this reason, the amplifier fails to operate near a saturated output, which would allow the amplifier to maintain a relatively high efficiency. This situation (i.e. amplifier barely operates near a saturated output) forces the amplifier to be used at a low efficiency.
To solve this problem, a variety of techniques have been proposed for improving the efficiency of an amplifier while maintaining the linearity. One of such techniques is a Doherty amplifier. The Doherty amplifier, which is aimed at improving the efficiency of a high output power amplifier, delivers a combination of outputs of a carrier amplifier and a peak amplifier which are different in operation class from each other. The basic operation and configuration of the Doherty amplifier was published in an article entitled “A New High Efficiency Power Amplifier for Modulated Waves” by W. H. Doherty, 1936 Proc. of IRE, Vol. 24, No. 9, pp 1163-1182, and thus is well known in the art.
The Doherty amplifier operates with the carrier amplifier maintaining saturation near saturated output power, a higher efficiency can be provided than in normal class-A and class-AB amplifiers as a whole even if the Doherty amplifier delivers an output with a backoff taken from the saturated power. The backoff refers to the difference between average output power and the saturated power. A large backoff state thus refers to a state in which the average output power is smaller than the saturated power.
The general operating principles of the Doherty amplifier are well known, for example, from an article entitled “Advanced Technique in RF Power Amplifiers” by Steve C. Cripps, Artech House 2002, pp 33-56, relevant to the present invention will be described below.
A combiner circuit for combining the output of the carrier amplifier with the output of the peak amplifier is made up a transformer or an impedance converter, and often includes a transmission line of one-quarter wavelength when it handles a signal in a micro-wave band. For an ideal operation of the Doherty amplifier, the impedance of a load, as viewed from an output combination point, is typically set to Z0/2, where Z0 represents the characteristic impedance of a transmission line. Typically, Z0 is set to 50 Ω in high frequency circuits.
As a circuit for covering the load impedance Z0/2 to the characteristic impedance Z0 in order to accomplish impedance matching, in a high frequency band, a transmission line of one-quarter wavelength is used between the combination point and output. As an example of such a circuit, a combiner circuit implemented in the micro-wave band was published in an article THE MICROWAVE DOHERTY AMPLIFIER” by R. J. McMorrow et al., 1994 IEEE MTT-S Digest (TH3E-7), pp 1653-1656).
Amplifiers have also been increasingly required to produce higher power, and this requirement has conventionally been met by operating a plurality of amplifiers in parallel to produce higher power. Well-known methods for operating a plurality of amplifiers in parallel include an in-phase distribution combination type using a Wilkinson distribution combiner, a push-pull type, a balance type using a hybrid circuit, and the like. For example, an article “MICROWAVE TRANSISTOR AMPLIFIERS Analysis and Design second edition” by Guillermo Gonzalez, pp 327-333, Prentice-Hall, 1997, a general method of operating a plurality of amplifiers in parallel employs the balance type using a hybrid circuit, a Wilkinson distribution combiner, and the like.
From the foregoing, it is thought that Doherty amplifiers, having a high efficiency, are effectively operated in parallel for producing a higher output in order to satisfy the requirements for linearity and high efficiency and to accomplish a higher output.
As described above, in general the ideal impedance of a load, viewed from a Doherty amplifier, is different from characteristic impedance Z0 of a system, and is one half the characteristic impedance, i.e., Z0/2. Therefore, for operating a plurality of Doherty amplifiers in parallel in a push-pull configuration or in a balanced configuration, a circuit is required at the output to convert impedance Z0 of a combiner for combining outputs of the plurality of Doherty amplifiers, to load impedance Z0/2 of the Doherty amplifiers.
As an example of the parallel operation of Doherty amplifiers, “RF HIGH POWER DOHERTY AMPLIFIER FOR IMPROVING THE EFFICIENCY OF A FEEDFORWARD LINEAR AMPLIFIER” by Kyoung-Joon Cho et al, 2004 IEEE MTT-S digest (WE6C-3), pp 847-850 has proposed a balanced-type amplifier which couples two Doherty amplifiers using a hybrid circuit for parallel operation. FIG. 1 is a circuit diagram illustrating a conventional circuit for operating Doherty amplifiers in parallel. Referring to FIG. 1, at the output of each of Doherty amplifiers 51, 52, there is connected quarter-wavelength impedance conversion line 53, 54 for converting the impedance from Z0/2 to Z0. Hybrid 55 is connected at the outputs of quarter-wavelength impedance conversion lines 53, 54 that for combines the outputs of two Doherty amplifier 51, 52. These components make up a balanced amplifier based on the Doherty amplifiers that operate in parallel.
However, the foregoing prior art example suffers from the following problems.
The conventional combiner circuit illustrated in FIG. 1, presents a problem of larger signal transmission loss and lower amplifier efficiency due to the extended transmission line from each Doherty amplifier to the output.
Further, since the impedance conversion lines and power combiner such as the hybrid circuit are both constructed by one-quarter wavelength transmission lines, the band is limited so that a desired operation can only be performed in a limited narrow band.
Furthermore, a terminator is required in the combiner both in a configuration using a hybrid combiner and in a configuration using a Wilkinson combiner. Thus, such a configuration is disadvantageous in point of a reduction in size and cost because the line portion including the impedance conversion line is large.